Method of preparing propellant grain



Oct. 29, 1968 w STANLEY ET AL 3,408,431

METHOD OF PREPARING PROPELLANT GRAIN Filed Dec. 21, 1965 INVENTORS. Will/21m 6. \Sfanley Edwin E More/lo United States 3,408,431 METHOD OF PREPARING PROPELLANT GRAIN William G. Stanley, Ogden Dunes, and Edwin F. Morello,

Whiting, Ind., assignors to Standard Oil Company, Chicago, [1]., a corporation of Indiana Filed Dec. 21, 1965, Ser. No. 515,389 11 Claims. (Cl. 2643) ABSTRACT OF THE DISCLOSURE This invention relates to solid propellants having a thermoplastic binder and more particularly to a method for improving the ballistic properties of said propellant. The method of the present invention is described herein as applied to the production of propellants containing ammonium nitrate is the principal oxidizing agent; however, this should not be deemed a limitation of the method.

In gas generation .for rocket usage, it is desirable in many cases that the gas alfording composition develop gas at a uniform rate; in the art this is spoken of as burning at a uniform rate. Ammonium nitrate compositions consist essentially of ammonium nitrate particles in an oxidizable organic material (binder), and various other additives such as catalyst for the promotion of comibustion, carbon, chemical stabilizer to reduce decomposition, etc. to improve the burning quality, and to utilize the excess free oxygen made available by the decomposition of the ammonium nitrate, into grains. These oxidizable organic materials may be any thermoplastic known to the art for use in propellant compositions.

The multi-component binder or matrix former commonly consists of a polymeric base material and a plasticizer therefor. Particularly suitable polymeric base materials are cellulose esters of al kanoic acids containing 2-4 carbon atoms such as cellulose acetate, cellulose butyrate and cellulose propionate.

The poly vinyl resins such as poly vinyl chloride and poly vinyl acetate are also good bases; styrene acrylonitrile is an example of a copolymer which forms a good base material; poly acrylonitrile is another suitable base material; as are polyamide resins (such as nylons).

The plasticizer component of the binder also, preferably, contains combined oxygen. The oxygen may be present in the plasticizer as an ether linkage and/or hydroxyl and/or carbonyl; also the oxygen may be present as an inorganic substituent, particularly a nitro group.

In general, any plasticizer which is adapted to plasticize the particular polymer may be used. A single plas ticizing compound may be used; more usually 2 or more compounds are used in conjunction; for example, acetyl triethyl citrate and triethyl citrate, etc.

The particular requirements with respect to use will determine not only the polymer but also the particular plasticizer or combination of plasticizers which are used. The precise amount of binder is dependent upon the type of material forming the binder as well as the require ments for the particular grain.

The requirement for a solid propellant that is suitable for military use is that it be ballistically stable after long periods of storage at temperatures between +160 F. to 65 F. [Many binders have been used to form a shaped solid propellant and include those listed above. The binder material used with the ammonium nitrate to form physically stable grains must be flexible to compensate for changes in volume of the ammonium nitrate as it passes from one temperature to another, since ammonium nitrate exists in several solid forms depending upon the temperature. The binder material must compensate for the changes in the volume in order that such changes will produce a minimum amount of voids and cracks in the grains. Production of fissures in the grain either internally or externally over the surface of the grain creates additional burning surface which results in unpredictability of the ballistic performance of the grain. Thus, it is necesasry to provide a binder material which will provide a shaped grain of satisfactory physical sensibility. Furthermore, such grain must be capable of being ignited at extremely low or relatively high temperatures after being subjected to variable storage temperature conditions and to burn evenly and at such a rate as to distribute the impulse energy in accordance with the service required.

Finely powdered ammonium nitrate contains about 20% or more by volume of void space, and this void space must be completely filled in order to obtain a shaped propellant grain of the desired physical characteristics. Moreover, additional void space is provided when using an inorganic compound as the catalyst, and the binder must not only fill the voids of the ammonium nitrate but also the voids present in the finely powdered inorganic catalyst material.

For military uses a propellant is desired which has non-detonating characteristics rather than the detonating characteristics of ordinary ammonium nitrate explosives. The burning characteristics of non-detonating explosives are dependent upon the temperature and pressure in the combustion chamber. An initially high temperature of the grain will cause the propellant to operate at a higher combustion pressure and thrust than will a cooler tem perature. The firing duration will be shorter, but the total impulse will not be changed significantly. This indicates the the initial temperature of the grain has a decided effect on the burning rate and that weather conditions have to be considered when exacting performance requirements are to be met.

The velocity at which a solid propellant is consumed during operation is called the burning rate. It is measured in a direction normal to the propellant surface and is usually expressed in inches per second. The burning rate may be expressed by the following relation, in which the influence of all performance parameters is small compared to the chamber pressure and the initial grain temperature.

The burning rate or velocity of propellant consumption 1- is usually given in inches per second; the chamber pressure p in pounds per square inch; a and n are constants. The constant a varies with the initial propellant temperature, and thus the burning rate is a function of the temperature of the grain prior to combustion. For most operations it is desirable that the value of r be as large as possible. The lower the value of n, the less is the detonating character of the decomposition of a gas producing composition and the more even and smooth is the burning rate of the propellant grain. Thus, a sustained thrust rather than a detonation is obtained by smooth burning of the grain.

The temperature sensitivity for different solid propellants is usually expressed as the percentage change of thrust per unit of temperature change. Temperature changes eifect the equilibrium pressure and the burning rate. The definitions of the temperature coefiicients are given by Sutton, Rocket Propulsion Elements (2d ed., 1958).

Here is the temperature sensitivity coefficient of equilibrium pressure at a particular value of K (K is the ratio of the burning surface to the throat area), expressed in percent pressure change per degree temperature change. Mathematically it is defined as the partial derivative of the natural logarithm of the equilibrium chamber pressure p with respect to temperature T. The other temperature sensitivity coefficient u refers to the change in burning rate r of a solid propellant with respect to temperature T at a particular value of chamber pressure 17 It is also known as the burning rate temperature coefficient, while 1r is known as the temperature sensitivity of pressure.

For most propellant applications, as low a temperature cofiicient as possible is desirable and even required for engineering design consideration. Lower pressure levels over a given temperature level allows a sizable weight savings for most missile applications.

In view of the importance of a low temperature coefficient to improving propellant performance in military applications, the use of the present invention is highly desirable in the production of propellant grains.

It has now been discovered that the desirable ballistic properties mentioned above may be attained by the process of this invention.

A general description of the invention is as follows:

Raw propellant is introduced into a chamber which is adapted to allow pressure to be exerted on the propellant, and the pressure is applied to said propellant. The chamber is provided with an orifice through which the propellant is extruded. A receiving chamber is provided into which the extruded propellant will flow. Both orifice diameter size and propellant temperature are critical to producing the desired elfect. The extruded propellant may then be formed into a shaped grain immediately or at a later time, by means known to those in the art, e.g., by compression molding. In the past, some improvement in temperature coefficient was obtained by blending the propellant or shearing, which is referred to in the art as working.

However, the prior art methods were not uniform with respect to working the propellant. It is known that the binder must wet the ammonium nitrate crystal, and this is usually accomplished by said shearing technique. The blending or milling randomly shears the binder to create only minor improvement in temperature coeflicient as compared to the present invention which produces a uniform shearing of the binder. It is essential, however, that when extrusion is to be used, several factors must be considered in order to optimize the desired effect. These factors are: the type of propellant used, and more particularly the viscosity of the propellant; the particle size; the orifice diameter size; the shape or configuration of the orifice; the temperature of the propellant; the temperature of the propellant during extrusion; the pressure applied to extrude; and the extrusion velocity. A proper relationship between the above-mentioned factors is essential to obtain a maximum working or shearing without a degradation of the physical properties of the propellant; i.e., without a change in such properties as tensile strength to a breakdown of binder structure. Another limitation is that the binder must be fluid but should not be at such high temperature as to create the danger of autoignition of the propellant.

Therefore, if the fluid has a high viscosity, it is desirable to extrude through a larger orifice or heat the propellant to a higher temperature.- t is also necessary to control oxidizer particle size, since extrusion through a smaller orifice is likely to breakdown coarseparticles. The burning rate is directly proportional to oxidizer particle size, and in a given system a smaller particle will give a higher burning rate. If a higher burning rate is desired, then a smaller orifice may be used.

The orifice is preferably a smoothly contoured nozzle with a converging entrance. The smallest opening in the nozzle should be between of an inch of A of an inch. Of course, depending upon the other variables, it may be larger or smaller. If the nozzle has a larger land, that is the portion of the cylindrical part of the nozzle (the parallel sides), the orifice may be larger than would normally be used. If the orifice were merely an opening in the chamber into which the propellant is placed, then there may be too much working or shearing depending upon the size of the orifice. The pressure must also be regulated carefully so as not to create the undesirable eifects mentioned above. Since the temperature of the propellant greatly increases as is the extruded, it is essential that it be maintained below the auto-ignition point. If these variables are not carefully regulated, there may be too much working thereby creating the degradation of the binder, or too little working thereby giving no improved ballistic properties. For a propellant containing ammonium nitrate as the principal gas producing component, an initial temperature of between and 125 F. is preferred. An orifice of between of an inch is also preferred. One further point, however, is that as the temperature is increased the viscosity of the propellant is lowered and less working or shearing would be expected in an orifice of a given size. The pressure should be at least 2,000 p.s.i. but may be as high as is economically feasible. Keeping in mind, of course, that an excessively high pressure will create high temperatures because of a friction and also may create too much working. For the ammonium nitrate system, a pressure of 10,000 psi. with a temperature of between C.118 C. and an orifice diameter size of A is preferred. The following examples are given by way of illustration only and are not to be construed as limiting the scope of the invention.

The results of the tests reported herein below were first determined by the burning rate as calculated in a Crawford bomb and then calculating the other values by use of the equations set forth above.

Example I A propellant was prepared in a sigma blade mixer by concurrent mixing and heating the following formulation:

The propellant was then heated to a temperature of 110 C. and extruded through a Ms" orifice at 10,000 p.s.i. pressure.

The temperature coefficients for the above propellant were calculated before and after the extrusion and are listed below in Table I.

TABLE I Before Extrusion After Extrusion (peroent/ F.) (percent/ F.)

Example II The propellant ingredients of the formulation of Example I were heated to a temperature of 118 C. and extruded at a pressure of 10,000 p.s.i. through an orifice the size of which is indicated in Table II.

TABLE II Orifice Dia. 11,000 up 1rk Not extruded 0.074 0.17 0.39 Extruded thru 0.087 0.32

Example III The procedure of Example II was followed except the temperature of the propellant was 115 C.

By way of example, reference is made to accompanying drawing which illustrates a specific embodiment of the invention. The figure shows a cross-sectional view of a device suitable for extrusion of propellant. In the figure, propellant is introduced into upper chamber 11, wherein pressure is exerted upon said propellant by means of ram 12, thus forcing said propellant through orifice 13 contained in plate 14. The propellant then flows into receiving chamber 15.

What is claimed is:

1. A method for improving the ballistic properties of a propellant grain comprising a propellant composition containing an oxidizer and a combustible thermoplastic binder material, which method comprises, heating said propellant to a temperature of between 100 C. to 125 C.; extruding said propellant through an orifice of between to inch in diameter at a pressure of greater than 2,000 p.s.i.; and forming a shaped grain from said extruded propellant composition, said thermoplastic binder material being fluid, and at such viscosity at said temperature to be adapted to be sheared without degradation of the physical properties of said binder.

2. The method of claim 1 wherein the temperature is in the range of 115 C. to 118 C.

3. The method of claim 1 wherein the orifice has a diameter of A: to A inch.

4. The method of claim 1 wherein the pressure is 10,000 p.s.i.

5. The method of claim 1 wherein said oxidixer is ammonium nitrate.

6. The method of claim 5 wherein said thermoplastic binder material is cellulose acetate.

7. A method for improving the ballistic properties of a propellant composition containing an oxidizer and a combustible thermoplastic binder material, said thermoplastic binder material being fluid and at such viscosity 5 at said temperature to be adapted to be sheared without degradation of the physical properties of said binder which method comprises:

(a) heating said composition to a temperature of between 100 C. to 125 C.; and

(b) extruding said composition through an orifice of between to /1 inch in diameter at a pressure in excess of 2,000 p.s.i.

8. The method of claim 7 wherein said oxidizer is ammonium nitrate and said thermoplastic binder material is selected from the group consisting of cellulose esters of alkanoic acids containing from 24 carbon atoms, polyvinyl resins, styrene acrylonitrile, polyacrylonitrile, and polyamide resins.

9. The method of claim 8 wherein said thermoplastic binder material is cellulose acetate.

10. In the method of preparing a propellant grain comprising a propellant composition containing an oxidizer and a thermoplastic binder material, by molding said composition, the improvement comprising:

(a) heating said composition to a temperature of between l00 C. and 125 C.; and

(b) extruding said composition through an orifice of between A to A in. in diameter at a pressure in excess of 2,000 p.s.i., prior to said molding.

11. A method for improving the ballistic properties of a propellant composition containing an oxidizer and a combustible thermoplastic binder material, said thermoplastic binder material being fluid and at such viscosity at said temperature to be adapted to be sheared without degradation of the physical properties of said binder which method comprises:

(a) heating said composition to a temperature of between 100 C. to 125 C.; (b) extruding said composition through an orifice of between A to in. in diameter at a pressure in excess of 2,000 p.s.i.; and (c) molding said extruded composition.

References Cited UNITED STATES PATENTS 2,926,386 3/1960 Hutchinson 264-3 2,936,176 6/1960 Adelman 264-3 3,252,369 5/1966 Bartley et a1. 264-3 L. DEWAYNE RUTLEDGE, Primary Examiner. 

